1. Field of the Invention
The present invention relates to an image capture apparatus suitably applied to a video camera or the like, a lens apparatus to be mounted on such an image capture apparatus, and a method for correcting image quality.
2. Description of the Related Art
In general, optical lenses may cause a phenomenon called aberration, which causes an unfavorable colored portion, an out of focus, and an image distortion when an image is formed. For example, one kind of aberration is known as a lateral chromatic aberration (chromatic aberration of magnification) by which a blur occurs on the boundary area of a subject image. The lateral chromatic aberration is caused such that, when an image is formed, rays of red (R), green (G), and blue (B) passed through a lens have focal positions varied in the direction perpendicular to an optical axis depending on wavelengths. FIG. 1 illustrates an appearance of an image formation, in which rays of R, G, and B passed through a lens have focal positions varied in the direction perpendicular to an optical axis. A degree of displacement varies depending on characteristics of a lens and a zoom position, focus position and iris condition of an image capture apparatus using such lens. FIG. 2 shows an example representing a relationship between an amount of displacement of focal positions and the zoom setting.
In FIG. 2, the vertical axis represents an amount of displacement in focal positions and the horizontal axis represents a zoom position (from wide to telescopic views). In FIG. 2, the focal position of G is used as a standard. Displacements of R and B are relatively represented with reference to G. In particular, a video camera used for shooting television programs may require a decrease in lateral chromatic aberration because it appears as a varied registration error.
A lens material such as fluorite which shows stable optical performance without a difference in focal lengths over the broad range of wavelengths may be used for reducing the lateral chromatic aberration, for example. On the other hand, the reduction of aberration can also be expected by the use of a combination of lenses made of materials with different refractive indexes. However, fluorite is expensive. If fluorite is used, production costs may increase as a result. Similarly, in the case of combining a plurality of lenses, production costs may increase all the same. Thus, a technique has been devised to correct the lateral chromatic aberration by carrying out image signal processing on digital image data captured by an image capture apparatus.
In addition to the aberration, a phenomenon of light falloff at edges, which causes an image degradation attributed to characteristics of a lens, has also been known. The “light falloff at edges” is a phenomenon in which an amount of light at the edges of a screen falls compared with the center thereof. Such a phenomenon may be caused by obstructing part of peripheral rays with a lens barrel. The degree of light falloff varies extensively depending on the zoom setting and focus and iris conditions of an image capture apparatus using the lens. FIGS. 3A and 3B are graphical representations respectively showing an example of a light intensity ratio of the center to the periphery of the lens under each state of normal or wide angle of view. In FIGS. 3A and 3B, the light intensities (%) are plotted on the vertical axis and the locations from the center to the corner are plotted on the horizontal axis. In each figure, two curves are represented. A lower curve is one obtained when the iris position is set to full-aperture and an upper curve is one obtained when the iris is narrowed.
FIG. 3A is a graphical representation of a light intensity ratio of the center to the corner of the screen when the zoom is set to a wide angle. FIG. 3B is a graphical representation of a light intensity ratio of the center to corner of the screen when the zoom is set to normal. The common phenomenon in both FIGS. 3A and 3B is that the larger the iris opens the more the light intensity of the periphery (corners) of the screen drops compared with that of the center thereof. In addition, as shown in FIGS. 3A and 3B, the reduction of the light intensity also varies with the state of the zoom position. The light falloff at edges can be prevented by enlarging the diameter of the lens. Alternatively, similarly to the correction to the lateral chromatic aberration, the light falloff at edges may be corrected by carrying out image signal processing on digital data obtained by shooting.
Japanese Unexamined Patent Application Publication No. 2000-3437 (JP 2000-3437 A) discloses the correction of a decrease in image quality attributed to the aberration of a lens with respect to digital image data obtained by a digital camera.